1. Technical Field
The present invention relates to semiconductor fabrication and more particularly to a device and doping method which dopes a silicon based semiconductor with boron without the formation of an undesirable boron rich layer (BRL) at the substrate surface.
2. Description of the Related Art
Boron is a common doping element employed in semiconductor devices. The doping of silicon crystal with boron has been performed by many methods. These methods include implantation of elemental or molecular boron ions. Boron doping by solid state diffusion can be performed from spin-on deposited boron doped glass films or boric acid, B2O3 films deposited by chemical vapor deposition (CVD) or plasma enhanced CVD processes.
Other solid state diffusion methods include B2O3 films formed at a silicon surface via reaction of BBr3 or BCl3 vapor with oxygen gas in a furnace tube, B2O3 films provided by oxidation of BN materials, and amorphous Boron films (α-boron) deposited by CVD (chemical vapor deposition) on an oxide free silicon at temperatures above 500 degrees C. Still other methods include the growth of a boron doped silicon base semiconductor layer on top of a substrate by molecular beam epitaxy (MBE) or epitaxial growth by chemical vapor deposition at atmospheric pressure or at a reduced pressure.
There are severe drawbacks associated with each of the above mentioned boron doping methods. For example, ion implantation introduces crystal damage as a consequence of energy transfer in the stopping process of incoming ions. This leads to the displacement of the lattice silicon atoms. Depending on the implantation parameters like ion dose, ion energy and dose flux and on the parameters of the subsequent thermal processes, such as heating rate, gas composition in the furnace, etc., the implantation damage may evolve into secondary extended defects like dislocation loops and stacking faults. These extended defects severely degrade the electrical properties of p-n junctions and the minority carrier lifetime in the bulk.
The diffusion of boron from chemical sources like BBr3, BCl3, B2O3 or a-boron films in direct contact with a silicon based semiconductor may result in the formation of a boron silicon compound phase layer at the silicon surface, called a Boron Rich Layer (BRL). The BRL is known to be SiB6. The thickness of the SiB6 layer is governed by parameters like the temperature and time of the diffusion process, the concentration of oxygen in the gas mixture in the case of vapor chemical source diffusion, etc.
For heavy boron diffusion, extended crystal defects can be introduced in the bulk of the wafer induced by stresses associated with the presence of BRL. This is particularly detrimental for solar cell production which requires Boron doping for the creation of back-surface fields in solar cells with a p-type substrate or p+-type emitters in solar cell with an n-type substrate, while maintaining a minority carrier lifetime at a high value.
The MBE method can produce a boron doped layer of high crystalline quality, however, in cases where the production cost is a key factor like in photovoltaic industry, this doping alternative is not feasible.